There are several methods that have been developed for mobile station (MS) location determining. Among others, Time Of Arrival (TOA), Time Difference Of Arrival (TDOA), and Enhanced Observed Time Difference (E-OTD) are the most potential ones. Some of this methods like TOA and E-OTD have already been potential candidates for the T1P1 specification work.
As far as the WCDMA (Wideband Code Division Multiple Access) system is in question, the dynamic range of coverage availability is a critical problem while applying existing positioning methods (methods for locating the position) in the system. Practically, the links between the mobile station (MS) and the base station (BS), which are essential for MS positioning calculation, aren't always available or the quality levels of the transmitted signals aren't in the requested level. That is the case e.g. in some rural area due to coverage unavailability or in urban and indoor area due to shadowing and the so-called NLOS (Not Line of Sight). In the worst case, a mobile station MS is not even within the coverage of any of the related base stations due to the so-called near-far problem which is caused by slow and fast fading and intra/inter cell interference in the system environment.
FIG. 1 illustrates a very typical problem of the cellular MS positioning in WCDMA system. In FIG. 1, three base stations BS1 to BS3 serving for three cells C1 to C3, respectively, are shown. The coverage of the corresponding cells of the base stations differ between a reduced cell's coverage and an extended cell's coverage, as indicated in FIG. 1. Furthermore, three mobile stations MS1 to MS3 are shown which are situated at different positions within and without the coverage of the cells. The positions of the mobile stations are indicated by an ‘X’ in the figure.
In order to determine the positioning of a mobile station the LOS (Line Of Sight) of three base stations are necessary for most of existing algorithms. As it is shown in FIG. 1, the mobile station MS1 has lost the LOS to all of the three base station BS1 to BS3. The mobile station MS2 isn't within the original coverage of any of the base stations, but not far away from the border. On the other hand, the mobile station MS3 is such close to the base station BS3 that the receiving signal with satisfied SIR (Signal to Interference Ratio) level from other base stations is almost impossible or it is contradictory with overall system performance. Indeed, the situation of the mobile station MS3 may be the most usual and critical situation concerning the MS positioning while algorithms like TOA, TDOA are applied.
In comparison with GSM, another specific problem is related to the transmitted power or the level of the interference due to MS positioning. The less the interference caused by mobile positioning is, the better is the capacity and overall system performance for the WCDMA system.
From system architecture point of view, there should be entity(s) that take care of the allocated power/interference for MS positioning purposes and its effect on the system's overall power/interference situation. Moreover, the functional allocation and their implication for the system interfaces should be determined.
The most potential solution for above-mentioned problems is using the DTX (discontinuous transmission) feature of base stations. For instance, in FIG., 1, for the case of the mobile station MS3, transmission of the serving base station BS3 can be stopped for certain time so that the mobile station MS3 does location measurements from signals of neighbouring base stations (here BS1 and BS2) during the idle slot.
However, even using this solution causes remarkable additional interference to the system and ultimately reduces the overall system capacity by changing the power/interference level in the system and making it unstable. Furthermore, in the case of the situation of the mobile station MS1 or the mobile station MS2 illustrated in the FIG. 1, this would be an even more problematic solution.